Foundations for Strawbale Buildings
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Q: I have a question regarding the foundations for a post and beam strawbale home. I specifically like the post and beam option because it appears to be much stronger and will be better received by the building officials, but I also like the post and beam option because of it's appearance when the beams are exposed on the inside surface of buildings. My question is what foundation options exist for a post and beam strawbale where the posts are on the inside of the bales? This approach doesn't appear to be common. Are there reasons why I should avoid it? Also if this approach is used what should be done to provide the lateral support which building codes require?

A: A wide range of foundation options can be used.  You'll have to get an engineer to evaluate your project and design the foundation accordingly.  This may seem too vague, but an engineer must evaluate your structure and local conditions -- seismic, soil conditions, frost depth, etc. A few ideas:
- Post and beam structures meet building codes if you use certified wood.  The posts carry the structural loads.
- One option is to buy a pre-engineered post and beam kit and wrap it in bales.
- Plastered bale walls provide lateral support, although your building officials will likely require additional strapping or tie wires.  Ask them what they require and/or work with an architect or engineer in your area with experience in SB construction.
- Be sure to insulate the foundation.  Heat will take the path of least resistance.
- Look for an engineer knowledgeable about low-cost, sustainable foundation options such as rubble trench, scoria-filled earthbags, pumice-crete, frost-protected foundations, etc.
- Placing the posts on the interior surface is quite common and also a good way to protect the wood.

Q: trying to find a formula for material ratio in bags for a earth bag foundation for a strawbale home. Specifically what materials and what ratio.

A: (Kelly) I recommend the use of washed 3/4 minus gravel to fill earthbags for strawbale foundations. This will keep any water from wicking upward and will provide a bit of insulation at the same time. Of course the bags need to be well protected from the sun and physical damage with a good plaster...and rather soon.

C: About the foundations, I downloaded a straw bale guide from Amazon. It states that the foundations can be from concrete blocks with rubble in the middle (the one I like best). As they don't use such concrete blocks here, I thought I would make a double concrete foundation each about 15cm wide and pour rubble (10cm) between the two, to provide drainage.

If you have space in the middle as you describe, consider filling the space with insulation such as lava rock.

Toe-up: The gravel-filled toe-up you describe is a good choice for raising the bales off the floor in case of a water leak or flood. It's often just 5-10 centimeters (2-4 inches) high. Don't confuse a rubble trench (which goes under the concrete foundation stem wall) with the toe-up.

C: What exactly is a toe-up? Is it the base plate that goes between the foundation and the bales?

The toe-up is the base plate (often filled with pea gravel for drainage) that goes between the floor and the bales.

Q: If I have a stem wall (required by Pima county code) do I still need toe-ups on top of that?

A: Raise the stem wall above the level of the finished floor and no separate toe-up is needed.  One option is to use earthbags filled with aggregate.

Q: Our land has almost no soil, but a lot of rocks. Our soil is officially (soil & water conservation dept. info) from 4" to 18" deep, and that small amount is mostly rocks barely held together by a little bit of dirt. That sits upon solid limestone/dolomite bedrock. For this reason, we plan to dig down only as far as we are able (probably about a foot at the site we have selected) and as wide as a 2-string bale; then back fill with gravel for a rubble trench foundation. Is a one-foot deep rubble trench good enough considering our ground conditions and the weight of a living roof (albeit a shallow one) on top of it? Alternative suggestions?

A: Clearing the soil and making a rubble trench foundation as described is a good idea.  All you need is to go down to solid or well drained rock.

Q: We had intended to pour a 4" tall, reinforced grade beam atop the gravel and top that with a full width, slip formed rock/cement stem wall (to lift the bales above grade enough to keep water out). We planned to put Styrofoam insulation outside the wall and below grade several inches since it would both help with any frost problems we might have from such a shallow foundation, and insulate the bottom of the bale wall a bit more than just rock and cement would do.) In retrospect, we feel that it may be overkill to bother with a grade beam when the stem wall will be as wide as the beam anyway, and equally strong. We also hate the idea of using something as un-environmental (and expensive!) as Styrofoam, but still need to deal with the whole insulation problem.

A: It sounds like you're mixing up the terms grade beam and stem wall.  Let's just call it a grade beam.  Pour an 8-10 inch grade beam on top of the rubble trench.  This raises the bales off the ground and allows for sloping the grade away from the building.  (4" isn't enough -- code says 6", plus you need drainage)  And rigid insulation on the outside is your best option.

Q: I am going to build with strawbale in Park County, Colorado. I am going with non load bearing system and post and beam framework. I want to utilize the rubble trench system under the bales. How deep should my trench be and should it be the width of my bales.

A: It's not clear exactly what you have in mind.  What will the bales sit on?  In general, a rubble trench should be the width of your bales and go to frost depth.

Q: My wife, Jamie and I are beginning a building project in Cass Lake Minnesota. We plan to build multiple buildings over the next few years - starting with a strawbale cabin this summer. I am very intrigued by your use of earthbags. The area that we are building in is all sand. Frankly I don't know how deep it goes but it goes a long ways for sure. It seems that a rubble trench with earthbags for the grade beam would be a great way to go. I want to use as little cement as possible, and I am very interested in the least toxic way to build.

A: Earthbags make an excellent foundation for straw bale walls.  In cold climates it would be best to use an insulating material such as scoria (volcanic rock) in the bags.  Bags of sand will wick away the heat rather quickly.  Another option is to use 2" of rigid foam insulation on the outside of the foundation, but this is more expensive and not as benign. You probably don't need a rubble trench, since the sand under the foundation would serve the same purpose (prevent drainage problems and heaving).